Suppose the US have decided to completely leave NATO, what steps would they have to take and how long can the process be?
Would it be different for other members of NATO should they wanted to leave?
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The document which codifies the NATO is the North Atlantic Treaty. Article 13 describes how a state can leave the NATO:
After the Treaty has been in force for twenty years, any Party may cease to be a Party one year after its notice of denunciation has been given to the Government of the United States of America, which will inform the Governments of the other Parties of the deposit of each notice of denunciation.
So the US would simply give a notice of denunciation to itself, inform the other members that they did and a year later they are officially out.
This article (as well as the articles 10, 11 and 14) mentions the United States as a member with a special secretarial role. It might be quite awkward to keep that after the United States left, so the remaining NATO members will likely have some amending to make.
But keep in mind that this is only the formal process. There are also a lot of practical considerations which might make leaving the NATO more complicated than that. The militaries of all NATO partners cooperate very closely. Breaking that up in just a year might be very challenging.
There would also need to be negotiations about what happens to any currently ongoing NATO operations. One interesting of many examples would be the Resolute Support Mission in Afghanistan. The Afghanistan war and its followup missions are NATO operations into which George W. Bush pressed the NATO after 9/11. It's questionable if the other NATO states would still be willing to invest money and soldiers into Afghanistan after the instigator of the whole mess left the treaty. And if they decide to still do so, it would need to be agreed on how the cooperation between US and NATO forces would work in practice.
And then there are quite a lot of US military bases in NATO countries. Whether or not the US is allowed to keep each base and for what price would also be a matter of negotiation. Many of the US military bases in Europe play crucial roles for the logistics of various US operations in the middle east. When these negotiations fail, the US forces would be forced to withdraw from these bases, which might in some cases be infeasible in such a short timespan.